This invention relates to a novel composition for forming a radiation absorbing coating and a radiation absorbing coating formed therefrom such as an anti-reflective coating. More particularly, it relates to a composition for forming a radiation absorbing coating which is particularly suited for forming an anti-reflective coating capable of effectively preventing reflection from a substrate of radiation to be used in manufacturing super-fine elements such as integrated circuit elements according to photolithography method and to a radiation absorbing coating such as an anti-reflective coating formed therefrom.
In the field of manufacturing integrated circuits, miniaturization of processing size in the lithography process has continuously proceeded in order to attain higher integration degree and, in recent years, technology enabling such fine processing of the order of sub-micron has been being developed. In such lithography process, a resist composition is coated on a substrate. The coating is then exposed using an exposure apparatus through a mask pattern to create a latent image of the mask pattern, and the latent image is developed using a proper developing solution to obtain a desired resist pattern. However, many of such substrates used have such a high reflectivity that, particularly when substrates with uneven surface are used, an exposure radiation having passed through the resist layer is reflected by the substrate to the resist layer again, thus problems of reflection notching, standing wave or dimensional unevenness of resulting resist pattern being caused.
In order to solve the problems, various methods have so far been investigated. For example, there are illustrated a method of dispersing or dissolving in the photoresist a dye having absorption at the wavelengths of exposure radiation (U.S. Pat. No. 4,882,260) and a method of absorbing exposure radiation having reached to the substrate by an anti-reflective layer formed on the substrate. As the latter method of forming an anti-reflective layer on the substrate, there have been known a method of forming an inorganic compound layer on a substrate by vacuum deposition, CVD method or the like (C. Nolscher, et al., Proc. SIPE, 1086, p. 242 (1989); K. Bather, et al., Thin Solid Films, 200, p. 93 (1991); and G. Czech, et al., Microelectronic Engineering, 21, p. 51 (1993)), a method of coating an organic polymer solution containing dissolved or dispersed therein a dye, a method of forming an anti-reflective layer by using a polymer dye wherein chromophores are chemically bonded to a polymer or using a polymer which itself has radiation absorbing property, and like method. These methods of using radiation absorbing polymers and radiation absorbing polymer materials for the methods are described in, for example, Japanese Unexamined Patent Publication Nos. H6-75378 and H6-118656, WO 9412912, U.S. Pat. Nos. 4,910,122 and 5,057,399, etc.
In forming the above-described organic anti-reflective coating, a photoresist-coating apparatus is often employed also as an apparatus for applying the anti-reflective coating composition. It is also desired in view of managing waste liquor that a solvent to be used for the anti-reflective coating composition be the same as that for the photoresist composition. However, in the case of forming the anti-reflective coating and the resist layer by using mutually the same solvent, intermixing between the anti-reflective coating and the resist layer is liable to take place upon applying a resist composition onto the anti-reflective coating already formed. In order to prevent this intermixing, it is necessary to introduce thermally cross-linkable properties into the anti-reflective coating. However, with some kinds of cross-linking agents to be used, the resist layer is adversely affected by the anti-reflective coating material containing the cross-linking agent to suffer footing or generation of scum, deformation of resist image, etc., thus failing to form a resist image with desired resolution and shape. In particular, where chemically amplified resists adapted for exposure radiation sources such as KrF excimer laser are used, the resists are liable to be adversely affected by the properties of anti-reflective coating, in particular, by acidic degree. Thus, improvement of the anti-reflective coating has been desired. In addition, in some cases, shape of resist pattern is changed in the interface between the resist and the anti-reflective coating depending upon the properties of the anti-reflective coating itself such as acidic degree. That is, with alkaline anti-reflective coating, tailing of a resist pattern is observed, whereas with acidic anti-reflective coating, resist pattern having undercut structure is formed. Thus, there has been desired an anti-reflective coating exerting less influences on a resist pattern.
An object of the present invention is to provide a coating composition which shows high anti-reflective properties, causes no intermixing upon applying thereon a resist, and causes no adverse influences on the shape of resist pattern and a method for forming a resist pattern using an anti-reflective coating having a proper acidic degree.
As a result of intensive investigations, the inventors have found that an anti-reflective coating having no conventional defects can be formed by incorporating in the composition for forming an anti-reflective coating a compound having two or more blocked isocyanate groups as a cross-linking agent, thus having achieved the present invention based on the founding.
That is, the present invention relates to a composition for forming a radiation absorbing coating which contains at least a radiation absorbing material and a compound having two or more blocked isocyanate groups.
In addition, the present invention relates to a radiation absorbing coating and an anti-reflective coating formed by using this composition for forming a radiation absorbing coating.
The present invention will be described in more detail below.
In the composition of the present invention for forming radiation absorbing coating, a compound having two or more blocked isocyanate groups is used which functions as a cross-linking agent. As the compound having two or more blocked isocyanate groups, compounds of the following (1) to (3) are particularly preferred:
(1) compounds represented by the following general formula (I) and having bis-isocyanate groups blocked with one of alcohols, phenols or oxime compounds:
xe2x80x83R1OOCHNxe2x80x94Xxe2x80x94NHCOOR2xe2x80x83xe2x80x83(I)
wherein X represents a straight-chain or branched alkylene group containing 1 to 30 carbon atoms, a divalent alicyclic hydrocarbon group containing 3 to 30 carbon atoms or a divalent aromatic ring group containing 4 to 30 carbon atoms, and R1 and R2 each independently represents an alkyl group containing 1 to 20 carbon atoms, an aromatic ring group containing 4 to 30 carbon atoms or an organic group having an imino group and containing 3 to 20 carbon atoms;
(2) polymers represented by the following general formula (II) and containing a recurrent unit having in a side chain an isocyanate group blocked with one of alcohols, phenols or oxime compounds: 
wherein Y represents a hydrogen atom, an alkyl group containing 1 to 3 carbon atoms or a halogen atom, Z represents an oxygen atom or NH, and R3 represents an alkyl group containing 1 to 20 carbon atoms, an aromatic ring group containing 4 to 30 carbon atoms or an organic group having an imino group and containing 3 to 20 carbon atoms; and
(3) condensation polymers between formaldehyde and phenyl isocyanate represented by the following general formula (III) and having an isocyanate group blocked with one of alcohols, phenols or oxime compounds: 
wherein R4 and R5 each independently represents an alkyl group containing 1 to 20 carbon atoms, an aromatic ring group containing 4 to 30 carbon atoms or an organic group having an imino group and containing 3 to 20 carbon atoms.
Additionally, in the above general formulae (I) to (III), the aromatic ring group includes a heteroaromatic ring group.
The compounds represented by the above general formulae (I) to (III) will be described more specifically below.
First, the compounds represented by the general formula (I) and having blocked bis-isocyanate groups can be prepared by reacting a corresponding bis-isocyanate compound with a desired blocking agent. As the bis-isocyanate compounds, there may be specifically illustrated such bis-isocyanate compounds as hexamethylenediisocyanate, 1,4-cyclohexanediisocyanate, toluenediisocyanate, bisisocyanatomethylcyclohexane, bisisocyanatomethylbenzene, bisisocyanatomethylethylbenzene, ethylenediisocyanate, etc.
As preferable examples of the alcohols to be used as the blocking agent, there are illustrated methanol, ethanol, propanol, butanol, ethylene glycol, methyl cellosolve, butyl cellosolve, methyl carbitol, benzyl alcohol, phenyl cellosolve, furfuryl alcohol, cyclohexanol, naphthyl alcohol, anthracene methanol, 4-nitrobenzyl alcohol, etc. Preferable examples of the phenols include phenol, substituted phenols such as cresol, xylenol, p-ethylphenol, o-isopropylphenol, p-tert-butylphenol, p-tert-octylphenol, thymol, p-nitrophenol, p-chlorophenol, etc., naphthol, etc., and preferable examples of the oxime compound include formaldoxime, acetaldoxime, acetoxime, 2-butanoneoxime, acetophenoneoxime, methyl ethyl ketoxime, 2,3-butanedioneoxime, cyclohexanoneoxime, diphenylglyoxime, benzaldehydoxime, etc. Of these, butanol, benzyl alcohol, 2-butanoneoxime, acetoxime, phenol, etc. are particularly preferred. However, the blocking agents are not limited only to these specifically illustrated ones.
The bis-isocyanate compounds blocked with these blocking agents undergo deblocking reaction at a temperature specific to each blocking agent to produce a highly reactive isocyanate group, thus cross-linking reaction proceeding. Additionally, mechanism of blocking with the blocking agent and deblocking thereof, the blocking agents, and bis-isocyanate cross-linking materials are described in, for example, xe2x80x9cKakyozai Handbookxe2x80x9d (compiled by Sinzo Yamasita and published by Taiseisha in 1990), p. 232-242. As the blocking agents (alcohols, phenols, and oxime compounds) or bis-isocyanate compounds used in the present invention, any of these known ones may be used.
The polymer containing a recurrent unit represented by the general formula (II) can be prepared by reacting an isocyanate group-containing polymer with the blocking agent of an alcohol, a phenol or an oxime compound. The isocyanate group-containing polymer may be a homopolymer or a copolymer additionally having other recurrent unit. The polymer having a recurrent unit represented by the general formula (II) may also be prepared by polymerizing the isocyanate group-containing monomer having previously been blocked with a blocking agent or by copolymerizing, if necessary, the blocked monomer with other monomer.
As the other monomer utilizable for forming the copolymer, any of known vinyl monomers may fundamentally be used that does not adversely influences cross-linking properties of the blocked isocyanate group. Preferred monomers include, for example, acrylic acid; methacrylic acid; acrylic esters such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, etc.; methacrylic esters such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, etc., styrene; alkyl vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, butyl vinyl ether, etc.; and the like. Of these copolymerizable monomers, acrylic esters and methacrylic esters are particularly preferred. The amount of the blocked isocyanate group in the copolymer may be not less than the amount necessary for completing the cross-linking. Kind and amount of the other comonomer may be selected depending upon the requirements for coating properties, stability, solubility, etc.
Average molecular weight of the polymer to be used in the present invention having a recurrent unit represented by the general formula (II) is generally 1,000-500,000, preferably 1,000-100,000.
As the blocking agent to be used for forming the polymer having a recurrent unit represented by the general formula (II), any of the alcohols, phenols and oxime compounds used for forming the compound of the above general formula (I) may be used. The condensation polymer represented by the general formula (III) can be prepared by blocking the isocyanate group-containing condensation polymer with the same blocking agent as described above. In this case, the oxime compounds are preferable as the blocking agents. Average molecular weight of the condensation polymer is generally 500-500,000, preferably 1,000-10,000.
Next, as the radiation absorbing material to be used in the composition of the present invention for forming a radiation absorbing coating, any of those that can absorb radiation at exposure wavelength to be used upon exposure of a photoresist may be used. As such radiation absorbing materials, there may preferably be used, for example, dyes conventionally known as radiation absorbing materials for formation of an anti-reflective coating, polymers having in a side chain a radiation absorbing group, polymers having a radiation absorbing group in a main chain, and the like. Specific examples of these radiation absorbing materials are found in, for example, U.S. Pat. Nos. 2,751,373, 2,811,509, 3,854,946, 4,609,614, 4,910,122, 5,057,399, 5,110,697 and 5,234,990, Japanese Examined Patent Publication Nos. H6-12452 and H6-64338, EP Patent 542008, U.S. Pat. No. 5,554,485, Japanese Unexamined Patent Publication Nos. H6-75378, H6-118656, H6-313968 and H7-82221, WO 97/07145, Japanese Unexamined Patent Publication No. H10-48834, WO 98/12238, Japanese Patent Application No. H9-146800, PCT/JP98/02234 and Japanese Patent Application No. H10-66626.
As the radiation absorbing material, those polymers and the composition thereof suffice which have enough absorption of radiation at exposure wavelength to be used as an anti-reflective coating. Preferable examples thereof include copolymers between a vinyl monomer having an aromatic ring such as a benzene ring, naphthalene ring, anthracene ring or the like in a side chain and a (meth)acrylic ester. As the specific examples thereof, there are illustrated copolymers represented by the following formulae (IV)-(VI) (wherein only copolymer components are shown). The copolymers may be ones containing (meth)acrylic ester monomers having a reactive functional group such as those of the formula (IV) or (VI), or may be copolymers containing (meth)acrylic ester monomer having a stable ester group such as those of the formula (V). With the former copolymers, the reactive functional group reacts with the aforesaid cross-linking agent having a blocked isocyanate group to form a cross-linking structure. And, with the latter copolymers, they do not react with the radiation absorbing material but the cross-linking compound molecules react with each other to cause self cross-linking and form a strong coating. 
However, as has been described hereinbefore, the radiation absorbing material to be used in the composition of the present invention for forming the radiation absorbing coating is not limited only to these specifically disclosed ones.
A proper film-forming resin may be added in order to improve film-forming properties of the composition for forming radiation absorbing coating, regardless of whether the radiation absorbing material and the isocyanate-containing compound to be used in the composition of the present invention for forming radiation absorbing coating have no film-forming properties or the radiation absorbing material have film-forming properties. As such resins, there may be illustrated those main-chain type polymers which have conventionally been used as base polymers for resist, such as novolak resin, hydroxystyrene-containing polymer, polyimide or the like. In this case, the polymers themselves may not have a substituent capable of reacting with an isocyanate group. When such polymers are used, insolubilization of the coating is attained by cross-linking between the blocked isocyanate compound molecules with each other or between the blocked isocyanate compound and the radiation absorbing material. Furthermore, cross-linking density may properly be adjusted by adding a low-molecular compound or a high-molecular compound having a substituent or substituents capable of reacting with isocyanate group such as hydroxyl group. As such compounds, there may be illustrated polyhydroxystyrene, novolak, hydroxyethyl methacrylate, bifunctional alcohol or an oligomer or polymer having carboxyl group.
In the composition of the present invention for forming a radiation absorbing coating, a solvent is used which can dissolve or disperse all components in the composition. As the solvent, any one that can dissolve or disperse each of the components in the composition may be used. Examples of such solvent include xcex3-butyrolactone, cyclohexanone, dimethylacetamide, dimethylformamide, dimethylsulfoxide, N-methylpyrrolidone, ethyl lactate (hereinafter abbreviated as xe2x80x9cELxe2x80x9d), methoxypropanol (hereinafter abbreviated as xe2x80x9cPGMExe2x80x9d), propylene glycol monomethyl ether acetate (hereinafter abbreviated as xe2x80x9cPGMEAxe2x80x9d) and the mixtures thereof. These examples are not intended to limit the solvent used in the present invention. Of the above-described solvent, xcex3-butyrolactone, cyclohexanone, EL, PGME, PGMEA and a mixture of PGME and PGMEA are particularly preferred.
In addition, to the composition for forming a radiation absorbing coating may be added a heat- or photo-acid generating agent, an aliphatic or aromatic sulfonic acid or a salt thereof with, for example, an amine. The heat- or photo-acid generators are those which are generally used in coatings, photoresists or the like. As the sulfonic acid, there are illustrated methanesulfonic acid, propanesulfonic acid, fluorine-containing alkylsulfonic acid, benzenesulfonic acid, toluenesulfonic acid, dodecylbenzenesulfonic acid, phenolsulfonic acid and naphthalenesulfonic acid. As the salt-forming amine, triethylamine, aminoethanol, diaminoethanol, ethylamine, propylamine, butylamine, benzylamine, etc. may be used.
In order to more strengthen cross-linking, there may be added generally used cross-linking agents such as melamine derivatives such as hexamethoxymethyl melamine, ethylene urea derivatives including ethylene urea, epoxy group-containing compounds, etc. Further, in order to adjust adhesion properties to a substrate and coating properties, there may be added known additives having conventionally been used upon formation of a radiation absorbing coating (e.g., anti-reflective coating) such as a surface active agent, a silane leveling agent, etc.
Additionally, although components of the composition for forming a radiation absorbing coating have been individually explained, the composition of the present invention may of course be that which is obtained by adding the blocked isocyanate group-containing compound of the present invention to a conventionally known composition for forming an anti-reflective coating (containing or not containing a cross-linking agent) not containing the blocked isocyanate group-containing compound, whereby properties of conventionally known anti-reflective coating can be improved.
The composition of the present invention for forming a radiation absorbing coating is applied on a substrate in a dry thickness of, for example, 300-5000 xc3x85 according to a conventionally known method such as spin coating, roller coating, dip coating or cast coating, and was baked on a hot plate or in an oven to evaporate the solvent and cause cross-linking, thus the coating being made insoluble for a solvent of a coating composition to be applied thereon. The baking temperature is generally 70-260xc2x0 C.
A photoresist composition is usually applied on the radiation absorbing coating such as an anti-reflective coating formed from the composition of the present invention for forming a radiation absorbing composition, thus a photoresist layer being formed. As the photoresist to be applied onto the radiation absorbing coating of the present invention, any of conventionally known positive-working or negative-working photoresists may be used. Typical examples of usable photoresists include a positive-working resist comprising a novolak resin and a quinonediazide photo-sensitive agent, a chemically amplified positive- or negative-working resist, a negative-working resist containing photo-sensitive double bond or of azide type. However, photoresists to be applied on the radiation absorbing coating of the present invention are not limited only to these illustrated ones. Further, needless to say, the coating to be formed on the radiation absorbing coating is not limited only to the resist layer.
The composition of the present invention for forming a radiation absorbing coating is particularly useful for forming a bottom anti-reflective coating upon manufacturing integrated circuits. In addition to the anti-reflective coating, the composition may be useful for forming an isolation layer for preventing mutual action between a resist and a substrate. Further, with stepped substrate, uneven surface can be flattened by applying the anti-reflective coating, thus the coating may function as a leveling layer. As a result, coating uniformity of the resist can be so improved on a stepped substrate on which the anti-reflective coating has been formed that fluctuation in pattern dimension can be suppressed.